docs: add FAQ documentation for common issues (#544)

- Add FAQ.md in English, Chinese, and Japanese
- Link FAQ from each language README
- Cover: PDF export, offline deployment, self-hosted models, image upload

docs/en/FAQ.md

@@ -0,0 +1,76 @@
+# Frequently Asked Questions (FAQ)
+
+---
+
+## 1. Cannot Export PDF
+
+**Problem**: Web version redirects to `convert.diagrams.net/node/export` when exporting PDF, then nothing happens
+
+**Cause**: Embedded Draw.io doesn't support direct PDF export, it relies on external conversion service which doesn't work in iframe
+
+**Solution**: Export as image (PNG) first, then print to PDF
+
+**Related Issues**: #539, #125
+
+---
+
+## 2. Cannot Access embed.diagrams.net (Offline/Intranet Deployment)
+
+**Problem**: Intranet environment shows "Cannot find server IP address for embed.diagrams.net"
+
+**Key Point**: `NEXT_PUBLIC_*` environment variables are **build-time** variables, they get bundled into JS code. **Runtime settings don't work!**
+
+**Solution**: Must pass via `args` at build time:
+
+```yaml
+# docker-compose.yml
+services:
+  drawio:
+    image: jgraph/drawio:latest
+    ports: ["8080:8080"]
+  next-ai-draw-io:
+    build:
+      context: .
+      args:
+        - NEXT_PUBLIC_DRAWIO_BASE_URL=http://your-server-ip:8080/
+    ports: ["3000:3000"]
+    env_file: .env
+```
+
+**Intranet Users**: Modify Dockerfile and build image on external network, then transfer to intranet
+
+**Related Issues**: #295, #317
+
+---
+
+## 3. Self-hosted Model Only Thinks But Doesn't Draw
+
+**Problem**: Locally deployed models (e.g., Qwen, LiteLLM) only output thinking process, don't generate diagrams
+
+**Possible Causes**:
+1. **Model too small** - Small models struggle to follow tool calling instructions correctly, recommend 32B+ parameter models
+2. **Tool calling not enabled** - Model service needs tool use configuration
+
+**Solution**: Enable tool calling, e.g., vLLM:
+```bash
+python -m vllm.entrypoints.openai.api_server \
+    --model Qwen/Qwen3-32B \
+    --enable-auto-tool-choice \
+    --tool-call-parser hermes
+```
+
+**Related Issues**: #269, #75
+
+---
+
+## 4. "No Image Provided" After Uploading Image
+
+**Possible Causes**:
+1. Model doesn't support vision (e.g., Kimi K2, DeepSeek, Qwen text models)
+
+**Solution**:
+- Use vision-capable models: GPT-4o, Claude 3.5 Sonnet, Gemini Pro Vision
+- Models with `vision` or `vl` in name support images
+- Update to latest version (v0.4.9+)
+
+**Related Issues**: #324, #421, #469